Great circle navigation instrument



July 24, 1951 L. E. GURNEY 2,561,794

GREAT CIRCLE NAVIGATION INSTRUMENT Filed Sept. 22, 1945 2 Sheets-Sheet l (x Kl 1 \NVENT'OR 6 {2 I9 16 14 Lawrence E Zia/"nay ATT C} R Elk" July 24, 1951 L. E. GURNEY GREAT CIRCLE NAVIGATION INSTRUMENT 2 Sheets-Sheet 2 Filed Sept. 22, 1945 60 $0 40 3O1IDIO \ill BOLD M-O I30 "0 110 H0 V Y Rm ww R o E? E5 m vmmm M lw Patented July 24, 1951 GREAT CIRCLE NAVIGATION INSTRUMENT Lawrence E. Gurney, Los Angeles, Calif.; Hazel '11. Gurney administratrixof said, Lawrence E.

Gurney, deceased Application September 22, 1945, Serial No. 617,952

3 Claims.

invention: relates to navigation;..and more particularly to navigation alongv a greatacircle route "between, points on the globe.v

It-is well understood that the mostdirect route between. twopointson the globe is one that followsuatgreat circle. Should the points .fallalong a meridian, the great circle route can be navigated by sailing due north or due south. But, when the points are on separate meridians (corresponding, to therdifference in longitude), then the. course must, change in a continuousmanner in ordertomaintain the ship on the great, circle. It is one of the objects of this invention'to makerit possible toascertain at once'whether a ship-is following the great circlecoursaas well as to ascertain the required bearing for mainta-hiin theiship on its course. In this manner, deviations of, the ship. from the desired course canxiba easily corrected.

It is. another object of this invention to .provide a-simple instrument for ascertaining the requiredtlatitude, longitude, and. bearingof the ship at every point of its great. circle, course.

It, is still another object of this invention to make itpossible readily to determine a newgreat circle course should they destination of the ship be altered, or should there be a material variation off course.

This invention possesseshmany other advantages, and has, other objects which may be made more clearly apparent from a consideration. of one, embodiment of the invention. For this purpose thereis showna iormin the drawings accompanying. and forming part of the present specification. The form willnow be described intdetail, illustrating the general principles of the invention;. but it to be understood that this detailed description is not to be taken in a limiting sense, since the scope of this invention is: best defined by the appended claims.

, Referrin to the drawings: Figure 1 is adiagram of great circle arcs and spherical triangles useful for explaining the invention; Fig. 2 is a diagram of a planefigure'further illustrating the application of the invention; and I Figa3- isa=planviewof an instrument-incornporatingrxthe inventionsome of the scale grad- :uations bein omitted. so as to simplify the figure. "Fig; l representsrin orthographic projection, thexgreat'circle route 2 between the points A ;md#B:locatedi onstthe" sphere. Although thedis- :cussion willrbe' quite general, inthe present in,- sstance: thisgreat circle route is shown ascom- "prised wholly in the northern hemisphere.

is assumed, of course, that the latitudes and iongitudes of the places A andB are known. The equator is indicated by the straight line I.

Given the longitudes and latitudes of these two places, it is possible to calculate for this particular route the angle representedby the are 1 between the pole N of the sphere and a point C, this pointC being the intersection of the great circle route land the great circle 3 which passes through the pole N, and is the normal to the great circle route 2. In other words, the are p divides the spherical triangle ANB into two spherical right triangles "ACN and NCB. The great circle 3 correspondsto the meridian of the point'C. The are 1 corresponds to the altitude .of the spherical triangle ANB. Point C may also be designated asthe crest of the great circle route 2. i

Since the latitudes and longitudes-of the places A andB are known, enough of the parts of the spherical triangle ANB are'known to find any of its remainin parts; Thus, it can readily be proved, by an application of spherical 'trigonoinetry', that the following relation holds:

tan p=cos M.cot.LA (l) where pcorresponds to the. are shown in Fig. 1; where M is the angle between the'arcs AN and NC; and where LA is the latitude of the place" A.

Th angle M 'is readily obtainable from the'relationship tan M :cot Lmtan LB cosec R-cot R (2) angle: M by the aid'of Formula 2 above, andtby the further obvious relationship:

A +M=' where A and M are the longitudes, respectively, of places C and A. Now, let. itbeassumed that it is desired to check the latitud and longitude of a ship, airplane, or other vehicle traveling between the places A'and B on the great circle route 2. By appropriate astronomical observations, the latitude or longitude of the place where the ship is located is determined. By the aid of this inven-- tion it is possible very quickly to determine whether the ship is on thegreat circle course 2, or whether it is off that course, by determining the corresponding longitude or latitude of that point on the great circle which has the observed latitude or longitude.

For the purposes of discussion, the point P can be assumed as falling on the route 2, formin a spherical right triangle PNC'. The angle between the sides and p can be designated by D, and the angle between the sides 0 and 11," can be designated as a. The angle 0. corresponds tothe bearing of the ship at the point P if it is following the great circle course.

In thespherical triangleN'PC, the side 1) is known. Accordingly, the following relations exist: I

to the length of the line 40- 13 which is drawn from the point! perpendicular tothe line 4. One leg 6--ll is equal to sin p by construction.

Accordingly, in order to construct the right triangle 6-l 1-48, an arc can be struckwith the radius 6-i3 equal to the length of the line 49-18. Then the intersection [8 with line 8 forms one of the acute angles of the right triangle" 6-11-18.

cos D=tan p.cot C=tan ptan L (3) since L and c are complementary; and Y sin a= f p (4) 1 SID C If we assume the latitude L of the..point.P-, then the side 0 canv be determined by the obvious relation: I I v i V 0 plus L=90 Similarly, the angle D corresponds obviously to the diiference in longitudesofthe pointsP andC,for AEA 0 plus D A beingthe longitude of the point P By the aid of Equation 3, assuming either the latitude or longitude of the point P, the-longi=- v tude or latitude-can consequentlybe computed. Similarly, by the aid of-Equation 4, the bearing along the great circle route can be computed when the latitude of the point P is known.

By the aid of this invention, a simple graphic solutioncan be used inwhich the desired values are immediately read from appropriate scales. This graphic method can be readily explained by the-aid of Fig. 2. 1

-.;In this figure there are twomutually per-- pendicular lines 4 and 5. With their'intersection 5 as a center, aunit circle 1 is drawn, corresponding to a great circle of the sphere. Two lines 8 and 9 are drawn, parallel to the line 4 and spaced respectively by distances corresponding to sin p and tan p. These lines intersect the penpendicular line'ai at points ll and M respectively. i

Let us assume the latitude of any point P to be represente'd-bythe'arc Ill-'49 on this circle.

The point l0 corresponds to the intersection of Then, obviously,

CONTES- having the latitude L, the line H is extended until it intersects the line 9 at the point l2.

Then a line [3 from point I2 is" drawn perpendiofular to the line 4, having its base at point [6 on line 4.' This line 13 intersects the unit circle I at the point M. The radius l5, drawn from the center 6 to the point M, makes an angle D with line 4 that corresponds to the angle D of Fig. 1.

This can be readily deduced by the relationship of the plane right triangles of Fig. 2 and by the aid of Equation 3.

Thus, in Fig. 2, cos D'is equal to the length of line B--IB. By construction, the length of line E 4l;is equal to tan 7:; and the length of'line l24l (being equal to that of line 6-48 by con struction) is thus equal to cos D. Therefore, by trigonometric functions-the tangent of angle L is the ratio of line 41-42 to line 6--4I that is,

cosD

s In this'right triangle, the angle a of the bearing is truly represented by the angle between 'the lines 4 and fil8, as well as between the line 6-l8 and the side ll-48. That this relationship holds can be readily verified'bythe aid-of Equation 4. H Thus, sin a inFig. 2 is equal to the ratio of the lengthof line 6-4! to that of the line 6-48. But line 6-4! equals sin p by construction; and the line 5-48 equals cos'L or-sin 0. Therefore:

1' sin 12 sin a:

If the longitude of the point P -be' assumed, then the angle D can immediately be determined as'aforesaid, for the line 15 can be drawn to correspond with this angle D. Then thelatitude can be determined by drawing the line l3 perpendicular to line 4 through the point M. Then, from'the point [2 where line l3 intersects line 9, the line can be drawn. In this way,"the latitude L is graphically determined.

A simple instrument for'determining these values is illustrated in Fig. 3. In this figure there are two scale-carrying members which are pivoted together at 2|, on a center correspondin to the center 6 of Fig. 2. I

One of the members 22 is'shown as of gen- 1 erally rectangular configuration, and may be in the form of a paper chart '23 attached permanently ortemporarily to a rigid type backing.

I There is a center line 24 perpendicular to a line 26 corresponding to the line 4 of Fig. 2. Adjacent the top edge of member 22 there is a series of graduations 25, corresponding to angles the point falling upon the line 24. ,These gradua' tions correspondto a coslne scale; that is, from the center ,24, 'measured along the horizontal line 26, the graduation lines 25 correspond to the cosines of the angles marked thereon. Many more gra'duations for scale 25 would be drawn in a practical embodiment to permit greater accuracy in reading thescale; but, they lare omitted in order to simplify the drawing. 7 The scale extends from zero to corresponding to. a..cliameter of a unit circuit such/as"! of Fig. 2. H

There are. also indicated two horizontal lines 21 and 28, corresponding to the lines 8 and 9,of

Fig. 2. These lines are parallelto line 26 and are spaced therefrom respectively by the values'tan p and sine p; the scale forthese values is, of course, consistent with the cosine values of scale 25. In other words, the distance from the'90 point on scale 25 to the zero point is taken as unity; for this distance corresponds to the cosine of zero. For eachgreat circle course, the values of sin 32 and tan 1) can be calculated and. placed 75 on the chart 2-3." These trigonometric functions vchartior the next voyage.

thus correspond to-the 'positionofcrest C of the desired great circle. route2- To facilitate drawing these lines 21 and 28, scales:29dandw3filmay be provided adjacent the edges of the chart. These scales can-be .drawn with more graduations,.bu=t they are omitted in.- order to simpliiy the drawing.

lieu of such ierent: charts 23 could be prepared in advance for a .number o f' different voyages, with linesZ-I; 2B printed thereon. Then the proper chart can be placed in position'on thebaclring, and discarded at the end of the voyagepreparatory for a new Since there are only a'r'elatively' few major ports on the globe, the charts to be printed are not too numerous.

The other member 32 is shown as in the form of an arm, provided with an edge 33 radial with the center 2|, and preferably carrying graduations 34 marked. in degrees and in such a way that cosines of angles correspond to distances from the center 2|. Here, again, these graduations are consistent with scales 25, 29, and 30. Intermediate graduations are omitted to simplify the drawing.

Member 32 also carries the arcuate portion 35 marked in degrees, reading right and left from the central zero degree position which is aligned with edge 33. Finer graduations are again omitted for the sake of simplicity. Graduations 36 are intended to cooperate With the two indexes 3'! and 38, placed at right angles to each other, and respectively along center line 24 and line 26.

The manner of use of the instrument may now be set forth.

If the latitude L of a point on the great circle course 2 (Fig. 1) be assumed, then the member 32 is turned about the center 2| until the index 31 coincides with the angle of latitude on the scale 36. The edge 33 then crosses the line 21 at the point 39. The angle D is then read on scale 25. For latitudes in the southern hemisphere, the left-hand part of scale 36 is utilized.

The relationship of this setting with the triangle 6-|6l2 of Fig. 2 is apparent. The angle reading on scale 36 corresponds to the angle L of Fig. 2. The point 39 corresponds to point 12; and the distance from line 24 to point 33 corresponds to the cosine of angle D. This angle D, read from scale 25, can be utilized to obtain the longitude of the point P by simple addition; for angle D, as heretofore shown, is the difference in longitude of points P and C.

Conversely, the latitude can be obtained by a similar setting if the longitude be assumed. Knowing the longitude, the angle D can be obtained by simple subtraction. Then the member 32 is moved about its pivot until the edge 33 intersects line 21 at the proper scale line on scale 25, corresponding to the known angle D. The latitude can then be read off at index 31.

The bearing a. can be obtained, if the latitude L is known, by a setting to correspond to the triangle I'l-8l8 of Fig. 2. For this purpose, the scale 34 on member 32 is utilized. Themember 32 is moved until the angle L, as measured on scale 34. intersects the line 28, as at pointAZ. Then the bearing can be read oil the scale 35 at the point 33. This is apparent from the relationships set forth by Equation 4. The right triangle 2l-43-42 corresponds to the right triangle 6ll-l8 of Fig. 2. The angle at the apex 12, corresponding to the bearing a, is obviously graduations, I a ,number I of dif-- 6, indicated at the indexes, which-indicatesthe departure of edge 33 from1inei16. r i The'u'sesof theinstrument aremultiple. Grea circle positions can be checked. If 1 3. navigator finds. that the latitude and: longitude observations check with the great circle requirementsdetermined-by the instrument, he knows: he is. on the course.

Thezsail'ing course 'is also determined by the aid ofthe instrumentuand correspondsto the angled. l --If*the vesselwor ,otherwcraftis off. course, it is. possible: 1 easilyvto rechart a newrg reat circle course by calculating tan p and sin p anew.

The formorchart123 isia record of the voyage and can be kept as such if desired.

Since the point C (Fig. 1) corresponds: to that point on the course having the greatest latitude, the calculations of longitude and latitude of this point gives the crest position of the route. Furthermore, the route can be used even for points beyond the two places A and B, since these two places represent merely two positions of the complete great circle, for any point of which the de-,

vice may be used.

If desired, any number of correspondinglongitudes and latitudes can be tabulated for the great circle route, and then the points plotted on any kind of plane map, such as a Mercator projection. A smooth curve through the plotted points gives the true great circle positions.

The inventor claims: I

1. In an instrument for obtaining corresponding latitudes and longitudes of any position on a great circle route between two known places: a pair of members pivoted together on a center, one of said members having graduation lines corresponding to cosine values on each side of said center along a straight line through said center, representing a diameter of a unit circle, said cosine values being of angles from zero to one hundred eighty degrees, the center corresponding to ninety degrees, as well as one or more lines spaced from the said line representing a diameter by distances corresponding to th tangent function of the complement of the latitude of the crest of the great circle route; the other member having a straight edge radial of said center, the

, members being provided with a mutually coact- Til ing scale and index for indicating angular posi tions of the straight edge with respect to a line ing latitudes, longitudes, and bearings of any position on a great circle route between two known places: a pair of members pivoted together on a center; one of said members having graduation lines corresponding to cosine values on each side of said center along a line representing the diameter of a unit circle; said cosine values being of angles from zero to one hundred and eighty degrees; said one member also having graduation lines spaced from said diameter by distances corresponding to the sine and tangent of the angle between the pole of the earth and the crest of the great circle route; the other member having a cosine scale along a radius from the center; the members being provided with a mutually coacting scale and two indexes separated by ninety degrees for indicating the relative angular positions of the line representing the diameter and the cosine scale.

3. In an instrument of the character described for determining the bearing along any point of a lar positions of the cosine scale and the line through the center. Number Country Date 15 40,716 Germany Sept. 12, 1887 7 8 great circle route'between two known places: a REFERENCES CITED Pair of members pivoted together on center; The following references are of record in'the one of said members having a line parallel to a me of this patent: I w r 7 =1 line passing through the center, and spaced there- 3 from byv a distance corresponding to'the sine of UNITED STATES PATENTS the angle between the pole of the earth and the 1 i crest of the great circle route; a cosine scale on ggz gfgfi g d the other member, the center corresponding to 811625 Edmonds Feb 1906 the cosine of ninety degrees, said cosine scale 1 955392 Shimberg 1934 being along a radius from the center; the meml0 2302210 Graves v n Nov 17 1942 bers being provided with a mutually coacting f scale and indexfor indicating the relative angu- FOREIGN PATENTS 

